1. 1. Cream together softened butter and granulated sugar until light and fluffy (2-3 minutes).

2. 2. Beat in egg and vanilla extract until just combined.

3. 3. In a separate bowl, whisk together flour, baking powder, and salt.

4. 4. Gradually add dry ingredients to wet mixture, mixing on low until dough just comes together. Do not overmix.

5. 5. Divide dough in half, flatten into disks, wrap in plastic, and refrigerate for at least 30 minutes.

6. 6. While dough chills, prepare filling: combine packed light brown sugar, ground cinnamon, and 1-2 tablespoons milk to form a spreadable paste.

7. 7. Preheat oven to 375°F (190°C). Line two baking sheets with parchment paper.

8. 8. On a lightly floured surface, roll out one dough disk into a ⅛-inch thick rectangle (about 12×8 inches).

9. 9. Trim edges and cut into 12 rectangles (approximately 3×4 inches). Repeat with second disk to yield 24 rectangles.

10. 10. Place about 1 teaspoon filling on half the rectangles, leaving a border. Top with remaining rectangles.

11. 11. Press edges to seal, then crimp with a fork. Pierce tops with fork for venting.

12. 12. Arrange on prepared baking sheets. Bake 10-12 minutes until edges are lightly golden.

13. 13. Cool on sheets for a few minutes, then transfer to wire rack to cool completely.

14. 14. For glaze: whisk together powdered sugar and 2-3 tablespoons milk or cream until smooth and pourable.

15. 15. Drizzle glaze over cooled cookies and add sprinkles if desired. Let set 15-20 minutes. Serve.